Some electronic devices need non-flat screens and 3D shaped screens, such as a bended plane and folded edge for the reasons of design and technique. For improving of the strength of the protective glass of electronic devices, the glass needs strengthening through ion-exchange, normally by exchanging alkali metal ions having a smaller ionic radius in the glass with alkali metal ions having a larger ionic radius. During ion-exchange, since the alkali metal ions having a larger radius are restricted at the places where the alkali metal ions having a smaller radius are present, a compressive stress is formed in the surface layer of the glass. Generally, the glass is immersed into a molten metal salt such as KNO3 for ion-exchange, and the process is named chemical toughening. The temperature of chemical toughening must be higher than the melting point of KNO3 of 328° C.
The protective glass of electronic devices or touch screens generally has a higher glass transition temperature (Tg). The soda-lime glass normally has a Tg of about 530-560° C. The available glass in market suitable for chemical toughening generally has an amount of Al2O3 for formation of glass network facilitating ion-exchange and speeding up ion-exchange. However, the glass of this kind has a Tg of generally up to about 600° C. Al2O3 is a refractory oxide having a melting point higher than that of SiO2 and has a very high Al—O bond energy. The melting temperature of the glass will correspondingly increase when Al2O3 is present in the glass. The molar ratio of alumina to alkali metal ions in the glass is a key factor for determining properties of the glass. When the ratio is less than 1, it is more likely that alumina will enter into the glass network to replace non bridge oxygen, increasing the melting temperature and viscosity of the glass. Al2O3 also can make a contribution to increase the chemical stability of the glass.
The glass used for protecting touch devices is generally sodium-aluminosilicate glass having a high Tg and a quicker ion-exchange rate. The glass can have extremely high fracture strength through a suitable ion-exchange process and a Tg normally higher than about 580° C.
It is economical to use a sodium-aluminosilicate glass for production of a pure flat protective glass. However, when the 3-D shaped protective glass is required, an excessively high Tg becomes a disadvantage. A glass having a high Tg cannot be shaped through molding economically since the deformation temperature of the glass is higher than the Tg. A sodium-aluminosilicate glass has a deformation temperature normally higher than 600° C., which leads to decrease in lifetime of the mold and its coating significantly. It is expected in the art to have a glass having a Tg lower than 550° C. but a very high strength after chemical toughening. A glass having a lower Tg can be obtained by partially replacing sodium in the sodium-aluminosilicate glass with lithium. The strength of the glass can be greatly increased after chemical toughening, and a very high surface hardness can also be obtained at the same time. A silicate glass containing Li2O has a viscosity lower than a glass containing Na2O, if the content of alkali metal is the same. Therefore, the lithium-aluminosilicate glass has a lower molding temperature, and then inexpensive mold and coating material can be used.
The Tg of the lithium-aluminosilicate glass can be controlled to far lower than 550° C. through selection of components, therefore, the glass can be molded by a steel mold or aluminum mold with a nickel alloy coating. The other way around, an expensive mold of metal carbide or nitride, such as a tungsten carbide mold has to be used for a sodium-aluminosilicate glass having a high Tg.
On the other hand, the strength of lithium-aluminosilicate glass after a reasonable chemical toughening has the strength similar to that of the toughened sodium-aluminosilicate glass. The diffusion speed of lithium ions is faster than that of sodium ions; therefore, the period of toughening time for the lithium-aluminosilicate glass is shorter than that of the sodium-aluminosilicate glass. The lithium-aluminosilicate glass can be chemically toughened by sodium salts or potassium salts. Such flexibility of toughening confers more selections of toughening conditions and more potential on the lithium-aluminosilicate glass to meet the requirements of other properties and processes.
When the temperature is not distributed evenly in glass, the value of the thermal expansion of a glass is very important. A glass having a higher thermal expansion coefficient cannot be suitable for a quicker cooling speed, and a glass having a higher temperature after molding may generate cracks easily if exposing to surrounding atmosphere. Compared with a glass containing Na2O, the glass containing Li2O has a lower thermal expansion coefficient, and therefore, is suitable for a quicker molding speed.
During molding production, the costs and lifetimes of the molding material and the coating are critical to the total costs. The material of a common mold is tungsten carbide, and a metal mold such as a steel mold or a nickel mold or a steel/nickel alloy mold can also be used when the molding temperature is lower. General coating materials are made of noble metals such as platinum or iridium, and a rear earth metal coating, a DCL coating or a sol-gel coating can also be used under particular conditions.